Exposure to ultraviolet light, in particular, ultraviolet light B (UVB, wavelengths, 290-320 nm) is known to be a major causative factor in the development of skincancer. The precise mechanisms by which ultraviolet light induces tissue damage are not clear. It hasbeen suggested that cytotoxic reactive oxygen intermediates generated by UVB light in the skin induce DNA damage leading to cancer. In this regard, we have discovered that UVB light rapidly stimulates the production of hydroperoxides by mouse and human keratinocytes in culture in a process that does not require intact cells. Greater amounts of hydroperoxides are produced in calcium-differentiated keratinocytes when compared to growing keratinocytes. Purification studies using homogenates of keratinocytes have identified a major protein responsible for generating hydrogen peroxide in the cells in response to UVB light. This UVB light/peroxide generating activity requires oxygen and is eliminated by heat denaturation. Unexpectedly, sequence analysis identified this protein as catalase, an enzyme responsible for the degradation of intracellular hydrogen peroxide to oxygen and water. Our observations that, in response to UVB light, catalase generates reactive oxygen intermediates is highly divergent from the well known antioxidant functions of this enzyme. Our findings that inhibition of the hydrogen peroxide metabolizing activity of catalase with 3-amino-I ,2, 4-trizole or azide markedly enhances the ability of UVB light to generate hydroperoxides provides direct support for our hypothesis that catalase is potentially an important mediator of UVB light-induced oxidative stress and DNA damage in keratinocytes. To test this hypothesis, we will characterize the production of oxidants by catalase in response to UVB light and the catalase-mediated stress response in both growing and differentiated keratinocytes. We will also evaluate the role of catalase in UVB light-induced carcinogenesis using the mouse skin model. Our proposed studies will provide important insights into the mechanisms by which UVB light induces DNA damage in the skin as well as the role of catalase in UVB light-induced skin cancer.